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Obviously, the interface's job is to allow for the expedient transfer of data between the hard disk and the system. Since the hard disk is an important performance component in the PC, the performance of the hard disk interface is probably the most frequently examined aspect of an interface's overall quality. In fact, it is generally given far too much emphasis in my opinion, particularly by companies looking to sell hardware based on the speed of the interface.
I'm certainly not suggesting that the performance of the interface is not important. However, it's important to keep it in perspective. The main problem when it comes to looking at interface performance is that on modern systems, the speed of the interface is not the limiting factor to overall hard disk performance. If a given hard disk can't read data from its platters fast enough to saturate an interface of a given speed, going to a faster interface yields improvement only on reads of data already in the drive's internal buffer--which makes virtually no difference in overall, real-world performance. Despite this, companies try to claim that (for example) drives using Ultra DMA/100 are "50% faster" than those using Ultra DMA/66. In fact, at the time of this writing (late 2000) no IDE/ATA drive can saturate even a 66 MB/s interface, so going to 100 MB/s is essentially pointless. For a more complete discussion of this issue, see this page on the interface transfer rate specification.
Actually, I have devoted an entire section of the site to discussing hard disk performance, so you should look there for a lot more detail on this important subject. Of particular interest will be the page I referenced above on the interface transfer rate specification, as well as the general discussion of interface performance factors.
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